Optical network technology offers a competitive manner of delivering information and services in an effective manner. Synchronous Optical Networking (SONET) solutions are an important ingredient in creating a competitive edge. The international version of SONET is known as SDH (Synchronous Digital Hierarchy) which might use a 155 Mbps payload envelope. Two types of optical protection rings are known: opaque and transparent. Transparent shared protection rings are known to have two fibers or four fibers. Generally, a two-fiber optical shared protection can perform ring switching only, whereas, a four-fiber optical shared protection ring can perform either ring switching or span switching. Optical shared protection rings are very useful survivability schemes for optical networks because they have a simple topology, efficient bandwidth usage, excellent resilience to failures and fast switching times. Survivability schemes offer protection which addresses traffic restoration in the event of failures. The functionality and configuration details of optical networking solutions are generally governed by one or more standards including GR 1230, ITU-TG 841, and GR-253 CORE.
Traffic protection in the event of failure is critical for Optical networks (Metro, Core & access) Optical networks employ different methods to achieve Automatic Protection Switching (APS). This includes linear protection schemes (1+1, 1:1, 1:N) and ring protection schemes (UPSR/SNCP, 2 Fiber BLSR/MSRing, 4 Fiber BLSR/MSRing).
Two-Fiber BLSR:    A two-fiber BLSR requires only two fibers for each span of the ring. Each fiber carries both working channels and protection channels. On each fiber, half the channels are defined as working channels and half are defined as protection channels. The normal traffic carried on working channels in one fiber are protected by the protection channels traveling in the opposite direction around the ring. This permits the bidirectional transport of normal traffic. Only one set of overhead channels is used on each fiber.    A two-fiber BLSR supports ring switching only. When a ring switch is invoked, the normal traffic is switched from the working channels to the protection channels in the opposite direction. [GR1230 and G.841].Four-Fiber BLSR:    A four-fiber BLSR requires four fibers for each span of the ring. Working and protection channels are carried over different fibers: two multiplex sections (lines in SONET) transmitting in opposite directions carry the working channels while two multiplex sections (lines in SONET), also transmitting in opposite directions, carry the protection channels. This permits the bidirectional transport of normal traffic. The multiplex section overhead is dedicated to either working or protection channels since working and protection channels are not transported over the same fibers.    A four-fiber BLSR supports ring switching as a protection switch, as well as span switching, though not concurrently. Multiple span switches can coexist on the ring since only the protection channels along one span are used for each span switch. Certain multiple failures (those that affect only the working channels of a span such as electronic failures and cable cuts severing only the working channels) can be fully protected using span switching. [GR1230 and G.841].